User operated drug delivery devices are as such known in the art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicament, such as heparin or insulin. In particular, such devices have application, where a medicament is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, such devices have to fulfill a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. Moreover, the device should be suitable for recycling.
To meet these requirements, the number of parts required to assemble the device and the number of material types the device is made from need should be kept at a minimum.
The medicament to be dispensed by means of the drug delivery device is typically provided in a disposable or replaceable cartridge, such as a vial, an ampoule or a carpule comprising a slidably disposed piston to be operably engaged with a piston rod of a drive mechanism of the drug delivery device. By applying thrust to the cartridge's piston in distal direction, a predefined dose of the liquid drug can be dispensed and expelled from the cartridge.
Cartridges as they are commonly used with drug delivery devices are typically sealed by means of as sealing septum. Such a septum is usually designed as rubber stopper providing an air-tight seal but being pierceable by piercing elements such as injection needles or cannulae.
FIG. 5 shows an enlarged view of a typical configuration of a cartridge mounted in a cartridge holder. The cartridge 30 comprises a substantially cylindrically-shaped barrel 32 which in distal direction 48, pointing upwardly in the sketch of FIG. 5, is sealed by a penetrable or pierceable sealing element 42. The sealing element 42 almost covers the entire cross section of the head portion 40 of the barrel 32 and remains fixedly attached thereto by way of a beaded cap 41, typically made of aluminum.
A distal end face 24 of the cartridge holder is symbolized in FIG. 5 in cross section. Typically, the cartridge 30 is arranged inside the cartridge holder in such a way, that the septum 42 can be penetrated and pierced by an injection needle entering the cartridge holder via the central through opening 22 provided in the distal end face 24. Since a central through opening of the beaded cap 41 is typically aligned with the central through opening 22 of the distal end face 24, the piercing member may easily penetrate the septum 42.
The cartridge 30 is also sealed in proximal direction 50 by way of a piston slidably displaced inside the cartridge 30 in distal or proximal direction 48, 50. Since the medicament 34 contained in the cartridge may feature a certain viscosity and since injection needles designed to penetrate the septum 42 might be rather small in diameter, a substantial and considerable distally directed pressure has to be applied to the piston of the cartridge 30 in order to expel a predefined amount of the medicament via the distal end of the cartridge 30.
Even though the cartridge 30 may also be radially clamped between side wall section 25 of the cartridge holder, the distally directed dispensing force 52 may lead to a substantial compression 58 of the septum 42 in axial direction. Hence, during a dose dispensing action, the cartridge 30 may become subject to a distally directed displacement. After termination of a dispensing procedure, the elastic energy stored in the septum 42 may dissipate and the septum 42 may relax into its initial configuration.
Consequently, the vitreous body 32 of the cartridge 30 may become subject to a proximally directed displacement. Since the piston of the cartridge 30 is operably engaged with a drive mechanism of a drug delivery device, the piston may be hindered to follow the relaxation-induced proximal displacement of the cartridge 30. As a consequence, the piston moves in distal direction 48 relative to the vitreous body 32 of the cartridge 30, thus leading to a post-dispending droplet generation at the distal end of the injection needle. Also the central portion 44 of the septum 42 may bulge in distal direction 48 and may even extend through the central through opening of the beaded cap 41.
In case the injection needle has already been removed from the cartridge holder, the elastic relaxation behaviour of the septum 42 may lead to a considerable increase of fluid pressure inside the cartridge 30, which, during a repeated attachment of a piercing needle to the cartridge holder may lead to a respective droplet formation at the needle end.
Even though the cartridge would be sufficiently clamped in the housing, a respective fluid pressure may still built up when the septum 42 relaxes and axially expands in proximal direction 50.
Moreover, the axial displacement of the cartridge with respect to a cartridge holder is also disadvantageous in terms of dosing precision.
It is therefore an object of the present invention to provide a cartridge holder comprising beneficial properties in terms of dosing precision and reduced droplet generation. It is a further object of the invention to reduce mechanical stress and pressure acting on a septum of a cartridge during and after a dose dispensing procedure. The intended solution should be robust and failure safe. It should further be easy and cost-efficient to implement even with existing cartridge holder designs.